This invention relates to an improved process for the solid state polymerization of polyester. More particularly, the invention relates to a continuous solid state polymerization with a combined dryer-reactor stage.
Polymeric linear polyesters are those having fiber-forming properties and which are composed of a substantial portion (at least 85 percent by weight) of an ester of a dihydric alcohol and terephthalic acid. The polyesters are obtained by a polycondensation of terephthalic acid or an ester-forming derivative and glycol. Polyethylene terephthalate is the preferred polyester.
As known to those skilled in the art, polyesters are generally made in two stages. The first stage, or esterification stage, the diacid is reacted with a diol at elevated temperatures and pressures, with water being produced as a by-product. In the second stage, or polycondensation stage, the reaction is carried out until the intrinsic viscosity of the polymer melt reaches from about 0.20 or higher, for example, up to 0.80. At this time, the polymer melt is solidified by cooling and pelletized, or granulated, etc. The pellets are then subjected to a solid state polymerization reaction wherein a vacuum is applied at a temperature below the melting point of the partially formed polymer or prepolymer. Alternatively, the solid state polymerization can be accomplished by removal of the by-products, i.e. ethylene glycol and water, by passing an inert gas through the pellets or granules. Generally, the solid state polymerization is continued until the intrinsic viscosity, hereinafter referred to as I.V., reaches the desirable level, such as from 0.55 to 1.10, or even higher. It is known in the art that it is desirable at times to produce a polyester having an intrinsic viscosity of from 1.0 to about 1.2.
All measurements of I.V. were determined in a 60/40 phenol/tetrachloroethane mixed solvent at 30.degree. C.
Polyester prepolymers contain up to 0.7 percent moisture. Moisture is a by-product of a polymerization reaction by esterification. It is known that by subjecting undried polyester to solid state polymerization conditions, it will cause severe depolymerization or degradation which is accompanied by a decrease in intrinsic viscosity and an increase in carboxyl number of the polymer. To address this problem, it is known in the art that one must dry the prepolymer. It has been a standard procedure to dry the prepolymer to 0.005 percent or lower moisture content in a dryer at about 160.degree. C. or lower temperature before feeding it into the solid state polymerization reactor.
In the conventional continuous solid state polymerization process (See FIG. 1), the prepolymer is initially fed into a feed hopper (1). The prepolymer is then continuously fed to the crystallizer (2) in which the prepolymer is heated to about 160.degree. C. and crystallized to prevent sticking in the latter part of the process. The crystallized polymer then enters the dryer (3). Inside the dryer, the polymer moves down by gravitational force and comes into contact counter-concurrently with a hot dehumidified air stream. A dehumidifier (19) and an air heater (4) are required to respectively lower the dew point of the air to below -34.degree. C. and to heat the air to about 160.degree. C. before entering the bottom of the dryer. The residence time of the polyester in the dryer is about 2.5 hours, during which its moisture content is reduced to below 0.005 percent. The dried polyester then leaves the dryer at about 160.degree. C. and enters the preheater (5) in which the temperature is about 220.degree. C., which is approximately the temperature of the reactor. The residence time of the polyester in the preheater is about 30 minutes. The preheated polyester then enters the moving bed solid state polymerization reactor (6) through the top. Once inside the reactor, the polyester moves downward slowly by gravitational force or by mechanical means while a stream of hot inert gas passes upward to sweep away the reaction by-products, including ethylene glycol, water and acetaldehyde. The conventional reactor is equipped with an oil jacket (7).
The polyester inside the reactor is maintained at a near constant temperature by controlling the inlet temperature of the polyester, nitrogen or inert gas and hot oil at approximately the same value, which is about 220.degree. C.
The polymer product with a sufficiently high I.V. is discharged through the bottom of the reactor into a cooler (8) where it is cooled below 65.degree. C. so that it can be packaged and stored.
The nitrogen gas exiting the top of the reactor is about 220.degree. C. and is contaminated with reaction by-products. It is then cooled and purified in the nitrogen clean up system (9) or air clean up system before being recycled to the bottom of the reactor.
The recycled gas can be fed into a heater (10) and blower mechanism (20) prior to entering the bottom of the reactor.